Unfortunately, the dealers are not owned by the manufacturer. As a result, they have flexibility. For example, some time ago there was some discussion about the right oil for the second generation cars. It turns out that the dealer in question was using the oil from the third generation cars for servicing second generation cars. As most of the people will not keep their cars for ever, when problems start showing up, the dealer will come up with something like, the car is too old and we have a great finance deal to replace the car with a newer one. Most people are happy with that. As you noticed, the dealer is not too happy to explain what went wrong. You can check the paperwork and compare it with the manual. 4,600 miles is too short and the engine management light is bad for a 2 years old car under warranty.
I think commuting nowadays is more like stop start driving. The fact that you stop start in the middle of nowhere on the motorway, does not make it any different than city driving. The manual for the second generation cars mentions that mountain driving, towing, city, etc, classify as heavy use. As a result, the service intervals should be reduced. It has a figure which I cannot recall but I think it just halves the mileage between services. So, if you towing on a regular basis, this is something you should consider but check your manual for more details. For traffic driving, either on the motorway or in the city, if that is all the mileage the car does again this is not good. The key to a healthy DPF is smooth constant driving so that it settles and regenerates. But your engine management light indicates that the problem is something bigger than just the DPF regenerations.
Yes replacing the oil earlier will not do any harm. But at the same time, you have to inspect the car and make sure that it works properly and you do not get the engine management light on. In these cars, it means checking the injection system for leaks and checking the oil pick up to make sure that it is clear. You need a clear EGR and manifold as well to ensure smooth running. If everything is fine, then the oil changes should last more than 4,600 miles. Another factor to consider is how heavy you are with the throttle. If you floor it on a regular basis, you add extra stress to the DPF as more fuel goes through unburnt. Older cars with no DPF/EGR/etc could take more abuse but still even the newer cars are good enough. I have the second generation car with the full set of DPF/EGR/etc and it is now on 116K miles but still goes longer than the recommended service interval between oil changes.
https://www.theaa.com/driving-advice/fuels-environment/diesel-particulate-filters